In the precipitation of metal, in the form of metal cement, from a solution of the metal mixed with less noble metal as precipitant, i.e. the so-called "cementation", essentially similar steps always take place--without reference to what metals are involved in detail, whereby an exchange takes place between a more noble and a less noble metal, according to electrochemical series.
The term "cementation" is defined according to the work "Lexikon der Huttentechnik" (Lexicon of the Foundry Technique) by Luger, 4th Edition, 1963, Volume 5, Deutsche Verlagsanstalt Stuttgart, page 733, as follows:
Precipitation of a metal out of a solution by means of a less noble metal. The less noble metal possesses a more negative potential than the one to be precipitated, puts positively charged ions into solution, charges itself negatively thereby and discharges all cations with more positive potentials. Precipitation in metallic form. Thus, the potential difference in the different cations under the prevailing conditions is decisive.
The metal cement product is in the form of a finely divided particulate precipitate.
In cementation- or precipitation-processes of various metals, fundamental difficulties occur in that the precipitated metal, the so-called cement metal, coats the surfaces of the less noble metals used as precipitants and thus impedes or actually brings to a standstill the electrochemical reaction, that is, the ion exchange. Other hindrances for the course of the reaction result from the fact that either an oxide layer is formed on the surface of the metallic precipitant, or a coating of fine gas bubbles which are derived from electrolytic procedures results.
Thus, the precipitation of copper from a copper solution (prepared, for example, by leaching of a mined copper ore, or a copper oxide dump, or the like, with a weak inorganic acid, such as sulfuric or hydrochloric acids) takes place in the presence of iron on account of the electromotive difference between copper and iron according to the known exemplary reaction equations: EQU CuSO.sub.4 +Fe.fwdarw.FeSO.sub.4 +Cu; EQU CuCl.sub.2 +Fe.fwdarw.FeCl.sub.2 +Cu; or EQU 2 CuCl+Fe.fwdarw.FeCl.sub.2 +2 Cu.
The precipitated copper is termed "cement copper". The iron ordinarily is added to such solution in the form of sheet-iron scrap, cast iron bars, or iron sponge. In achieving precipitation, it is important that the surface of the iron be constantly kept clean; that is, the precipitated copper should be constantly separated from the iron, so that further copper may precipitate thereon. Towards this end, the iron pieces need to be kept constantly in motion.
It was previously usual in the zinc-wet-metallurgy field that the precipitating neutral solutions were freed from foreign metals dissolved therein, such as, for examples, Cu, Cd, Co or Ni. Thus, to a starting solution was added in agitators metallic zinc powder, and such mentioned impurities were thus precipitated out. Such known method however, has among others, the disadvantage that, first, for example, by means of spraying of metallic zinc, the zinc powder must be produced. In addition, the reaction progresses between the precipitating powder and the solution extremely slowly, with a reaction duration up to many hours, especially for the reason that a layer of zinc oxide forms on the zinc particles which decisively retards the progress of the reaction.
In the case of the known copper cement precipitation techniques, iron piece movement has been sought by various means. Thus, in one apparatus, an agitator device is installed in the precipitation container. In another apparatus, the iron is automatically introduced, or rather introduced into, automatically moved baskets or rockers made of copper or wood. In another apparatus, the precipitation is accomplished in rotating, acid-proof lined iron drums. When in such last apparatus, a certain acceleration of the course of the reaction can be attained, then, for example, with a precipitation drum of approximately 30 m.sup.3 of utilizable volume, it always takes about 1 hour or more until the copper is precipitated out to a desired copper residue content below about 0.1 g/l in the final decopperized solution, which is a convenient upper level in the cement copper production field for copper in a decopperized solution, as those skilled in the art appreciate.
German patent application No. G 105,641 VIa dated June 30, 1942 describes a device for batch copper cementation using a cylindrical upright container open at the top and provided with an oscillating drive. The container is pre-charged with iron pieces, and copper solution is injected at the container bottom during oscillations, and the decopperized solution is conveyed off through simple overflow from the open rim of the container. The rim carries a ring of filter stones to prevent iron particles from leaving the container.